This invention relates to an electric synchronous motor having a stator with electric windings and a rotor comprised by a spindle and a magnetic core. The core is formed by a number of annular magnetic discs piled on said spindle and separated electrically from each other by layers of insulating material.
It is previously known from U.S. Pat. Nos. 0,464,026 and 3,482,156 and from Japanese Patent Kokai No. 18950/88 (63-18950) to use a rotor with an axially laminated magnetic core consisting of a number of magnetic discs piled on a rotor spindle and kept together by an insulating resinous material.
This known technique, however, creates problems when designing motors for quick acceleration and retardation. To keep down the weight of the rotor, the rotation speed has to be increased, and to keep down the moment of inertia the diameter has to be small. Instead, the rotor has to be relatively long to keep up the performance of the motor. By using this previously known technique to form a magnetic rotor there will be a problem to obtain a good enough bending strength, a high flexing stiffness and a high critical rotation speed. Due to the fact that the magnetic discs according to the above mentioned patent are kept together by an adhesive which normally has a lower resistance to tension and splitting than to pressure strain, the bending strength of the rotor is reduced. This creates the risk that the weak rotor spindle will be subjected to all the strain.
The invention solves the above problems by using the rigidness and pressure load resistancy of the magnetic material of the discs. This is accomplished by prestressing the magnetic laminate to a stress level which is higher or equal to the occurring bending stresses caused by the load during operation. Hereby, the hollow cylinder formed by the laminate discs will act as if it were a solid body with the same stiffness and tension strength as for compressive load and bending.
Transferring torque from the active part of a long slim rotor to a small diameter spindle has also proved to be a serious problem for high performance motors in the past. Magnetic discs mounted with a pressfit or gluing work tend to loosen because of the twist deflection of the spindle in combination with temperature changes. Axial prestressing of the magnetic discs solves this problem as well.
The rotor design according to the invention is particularly suitable for so called slotless synchronous machines which permit high rotation speed but require a thick magnetic material in the direction of the magnetic flow to compensate for the wide magnetic air gap. A thick magnetic core, however, limits the spindle diameter.